The present invention relates to new cellulosic solutions which may be shaped to produce filaments, fibers, membranes, films, sponges and the like.
Previously used cellulose solvents were generally sulphuric acid and phosphoric acid, which tend to degrate the cellulose by causing severe hydrolysis, or complexes of heavy metals and amine compounds used in the cuprammonium process, which complexes are rather uneconomical to use because the solvent can not be recovered directly but only in an indirect and complicated manner.
The prior art has also known other solvent complexes or solvent mixtures for cellulose. Thus, the prior art has used ferric/tartrate complexes or mixtures of dimethylsulphoxide with dimethylacetamide or nitrogen dioxide, or sulphur dioxide and an amine. Because of major technical and/or economic problems, however, such complexes or mixtures have not lead to industrial spinning processes.
The most widely used spinning process includes an initial chemical conversion of the cellulose in order to produce a soluble intermediate product, cellulose xanthate, followed by a fresh chemical conversion to regenerate the cellulose. In such a process, significant consumption of chemical reactants, and especially a major loss of carbon disulphide, is involved.
Johnson et al, IPC Technical Paper Series of The Institute of Paper Chemistry, No. 5 (April 1975), report that they are able to dissolve cellulose, at very low concentrations, mainly 1 to 3% by weight/volume, in dimethylsulphoxide (DMSO) containing a large amount of formaldehyde relative to the amount of cellulose (on the order of 5 to 20 times the weight of the cellulose). Solutions of such low concentration are completely unsuitable for shaping on an industrial scale to produce filaments, fibers, membranes, films, sponges and the like, and the yield of the process of dissolving the cellulose is extremely low, because in most cases a large undissolved residue remains, which can represent up to 37% of the weight of the pulp, which is only employed at the rate of 1% relative to the DMSO. In addition, the high paraformaldehyde content interfers with the subsequent shaping operation. These authors teach that solutions of higher cellulose concentration can only be obtained from cellulose having a low degree of polymerization. As is known to the art, these celluloses of low degrees of polymerization are not found in the natural state, and can only be obtained by a prior chemical treatment of native cellulose, with consequential considerable increase in cost.
French Patent application No. 2,311,783, published on Dec. 17, 1976, claiming U.S. priority of an application filed on May 19, 1975, discloses a process for spinning solutions of cellulose in DMSO and formaldehyde. The French patent application describes solutions containing at least 0.8 parts by weight of aldehyde per part of cellulose, and 2 to 14% by weight of cellulose per volume of DMSO.
In order to dissolve a native cellulose, having a DP of at least 400, it is normally necessary to use a weight ratio of formaldehyde/cellulose of at least 1. At this formaldehyde content, however, it is difficult to effect further transformations of the solutions, and it is difficult to recover the majority of the formaldehyde used in the process.